Fix checksum code, some of the time it did not match Go/Python

[protolambda]

Checksums were wrong, here's some Go code to compare, along with examples of the JS checksums not matching, but only some of the time:

package main

import (
	"fmt"
)

func main() {
	// Go: https://github.com/golang/snappy/blob/2e65f85255dbc3072edf28d6b5b8efc472979f5a/snappy.go#L97
	// Python: https://github.com/andrix/python-snappy/blob/602e9c10d743f71bef0bac5e4c4dffa17340d7b3/snappy/snappy.py#L70
	
	// Did not match JS before
	// x = 3008619490
	// Old JS: ((((x >> 15) | (x << 17)) + 0xa282ead8)) >>> 0
	//   Result: 2726449535
	// New JS: ((((x >>> 15) | ((x & 0x7fff) << 17)) + 0xa282ead8)) >>> 0
	//   Result: 1783124351
	x := uint32(3008619490)
	checksum := uint32(uint32(x>>15|x<<17) + 0xa282ead8)
	fmt.Printf("Checksum: %d\n", checksum)
	
	// Did match js before
	// x = 1
	// Old JS: ((((x >> 15) | (x << 17)) + 0xa282ead8)) >>> 0
	//   Result: 2726619864
	// New JS: ((((x >>> 15) | ((x & 0x7fff) << 17)) + 0xa282ead8)) >>> 0
	//   Result: 2726619864
	x = uint32(1)
	checksum = uint32(uint32(x>>15|x<<17) + 0xa282ead8)
	fmt.Printf("Checksum: %d\n", checksum)
}

See https://play.golang.org/p/AzfhAHeV1bn for a live example.
Some previous checksums were incorrect.

Checksums test file also updated, I just set the UPDATE_EXPECTED env variable to get it updated.